Typically, as shown in FIG. 1, a wireless communication system 10 comprises elements such as client terminal or mobile station 12 and base stations 14. Other network devices which may be employed, such as a mobile switching center, are not shown. In some wireless communication systems there may be only one base station and many client terminals while in some other communication systems such as cellular wireless communication systems there are multiple base stations and a large number of client terminals communicating with each base station.
As illustrated, the communication path from the base station (BS) to the client terminal direction is referred to herein as the downlink (DL) and the communication path from the client terminal to the base station direction is referred to herein as the uplink (UL). In some wireless communication systems the client terminal or mobile station (MS) communicates with the BS in both DL and UL directions. For instance, this is the case in cellular telephone systems. In other wireless communication systems the client terminal communicates with the base stations in only one direction, usually the DL. This may occur in applications such as paging.
The base station to which the client terminal is communicating with is referred as the serving base station. In some wireless communication systems the serving base station is normally referred as the serving cell. While in practice a cell may include one or more base stations, a distinction is not made between a base station and a cell, and such terms may be used interchangeably herein. The base stations that are in the vicinity of the serving base station are called neighbor cell base stations. Similarly, in some wireless communication systems a neighbor base station is normally referred as a neighbor cell.
Duplexing refers to the ability to provide bidirectional communication in a system, i.e., from base station to client terminals (DL) and from client terminals to base station (UL). There are different methods for providing bidirectional communication. One of the commonly used duplexing method is the Frequency Division Duplexing (FDD). In FDD wireless communication systems, two different frequencies, one for DL and another for UL are used for communication. In FDD wireless communication system, the client terminals may be receiving and transmitting simultaneously.
Another commonly used method is the Time Division Duplexing (TDD). In TDD based wireless communication systems, the same exact frequency is used for communication in both DL and UL. In TDD wireless communication systems, the client terminals may be either receiving or transmitting but not both simultaneously. The use of the radio frequency (RF) channel for DL and UL may alternate on periodic basis. For example, in every 5 ms time duration, during the first half, the RF channel may be used for DL and during the second half, the RF channel may be used for UL. In some communication systems the time duration for which the RF channel is used for DL and UL may be adjustable and may be changed dynamically.
Yet another commonly used duplexing method is the Half-duplex FDD (H-FDD). In this method, different frequencies are used for DL and UL but the client terminals may not perform receive and transmit operations at the same time. Similar to TDD wireless communication systems, a client terminal using H-FDD method must periodically switch between DL and UL operation. All three duplexing methods are illustrated in FIG. 2.
In many TDD wireless communication systems, normally the communication between the base station and client terminals is organized into frames as shown in FIG. 3. The frame duration may be different for different communication systems and normally it may be in the order of milliseconds. For a given communication system the frame duration may be fixed. For example, the frame duration may be 10 milliseconds.
In a TDD wireless communication system, a frame may be divided into a DL subframe and a UL subframe. In TDD wireless communication systems, the communication from base station to the client terminal (DL) direction takes place during the DL subframe and the communication from client terminal to network (UL) direction takes place during UL subframe on the same RF channel.
When a client terminal is powered on, it may not have information about the frame timing of the base station. The client terminal may first perform power scan and select a suitable RF channel for performing synchronization with the base station timing. Therefore, in general the power scan may be performed by the client terminal when its timing is not aligned with that of the base station.
Orthogonal Frequency Division Multiplexing (OFDM) systems typically use Cyclic Prefix (CP) to combat inter-symbol interference and maintain orthogonality of the subcarriers under multipath fading propagation environment. The CP is a portion of the sample data that is copied from the tail portion of an OFDM symbol to the beginning of the OFDM symbol as shown in FIG. 4. One or more OFDM symbols in sequence as shown in FIG. 4 are referred herein as OFDM signal.
In addition to the purposes mentioned above, the CP is often used for frequency offset estimation at the receiver. Any frequency offset at the receiver relative to the center frequency of the transmitted signal will cause the phase of the received signal to change linearly as function of time. The two parts of an OFDM signal that are identical at the transmitter, i.e., the CP and the tail portion of the OFDM symbol, may undergo different phase change at the receiver due to the frequency offset. Therefore, the frequency offset can be estimated by performing correlation between the CP and the tail portion of the symbol. The angle of the CP correlation indicates the amount of phase rotation that is accumulated over the duration of an OFDM symbol. This accumulated phase rotation is then used for frequency offset estimation. There may be other impairments such as noise, fading and interference that may make the two parts of an OFDM symbol different. However, such impairments may be random in nature and may tend to cancel out during CP correlation computation. On the other hand, the phase offset between the two parts of the OFDM symbol tend to accumulate as each sample pair of parts being correlated may have on average the same phase difference.
The frequency offset at the receiver during initial synchronization may be very high. Furthermore, since the client terminal is not synchronized to any base station during initial synchronization, the OFDM symbol boundaries are not known to the client terminal. In wireless communication system deployments where frequency reuse is employed, the signals from several base stations may be superimposed. In some cases, the various base stations may not be time synchronized, i.e., the OFDM symbol boundaries for the different cells may not be time aligned. Even if the OFDM symbol boundaries are time aligned the propagation delays from different base stations to the client terminal may be different and therefore the OFDM symbol timing may not be time aligned at the client terminal receiver. Furthermore, in some wireless communication systems such as 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) or LTE-Advanced, an option of using different CP length exists and the exact CP in use may not be known a priori to the client terminal. Also, the different base stations whose signals may be superimposed may be using different CP length. The overall received signal scenario is illustrated in FIG. 5. In case of TDD systems since the same frequency is used for transmit and receive, at power up the client terminal may not be aware of the boundary between DL and UL. In case of TDD systems, the significant power difference between DL and UL may create challenges in performing frequency offset estimation and may lead to inaccurate frequency offset. A method and apparatus are disclosed that enable frequency offset estimation in presence of multiple interfering OFDM signals of different CP types and with or without time synchronization for all types of duplexing schemes.